1. Field of the Invention
This invention relates to a purification process for the efficient preparation of high-purity 2-acrylamido-2-methylpropanesulfonic acid.
2. Description of the Prior Art
2-Acrylamido-2-methylpropanesulfonic acid (hereinafter referred to simply as AMPS) to be purified according to this invention is a known compound which is prepared by the methods described in U.S. Pat. Nos. 3,506,707 and 3,544,597, British Pat. No. 1,090,779, German Offenlegungsschrift No. 2,523,616, and Japanese Patent Publication 30,059/75.
In a preparative method typical of those described in the above patent publications, isobutene and fuming sulfuric acid are allowed to react in the presence of an excess of acrylonitrile, which is also used as reaction medium, and the resulting AMPS is allowed to precipitate directly from the reaction mixture. The crude crystals thus obtained are washed with acrylonitrile and, if necessary, recrystallized from a solvent.
Beside being used as an agent for improving the dyeing property of acrylic or other fibers, AMPS and its homologs can be polymerized to form useful homopolymers and copolymers. These polymers are useful as polyelectrolytes and are known to be usable as flocculant, dispersant, adhesive and fluidity regulator, and there are many patents in these fields.
The crude AMPS crystals as obtained by washing with a solvent the crystals precipitated directly from the reaction mixture may be used in some of the abovementioned uses. However, in order to produce a polymer having a considerably high molecular weight, the crude crystals must be completely dissolved in a solvent and recrystallized therefrom.
For instance, the copolymer of AMPS and acrylamide for use as a mucilaginous material for papermaking and a flocculant must have a high molecular weight, and in order to produce such a high molecular weight copolymer, AMPS must have such a high purity as to be obtained by repeated recrystallization, otherwise no satisfactory result can be obtained.
The use of a copolymer of AMPS and acrylamide as a mucilaginous material for papermaking has been described in U.S. Pat. No. 3,772,142 and Japanese Patent Application Kokai (Laid-Open) Nos. 59,507/75 and 27,808/77.
Regarding the function of a mucilaginous material for papermaking, Japanese Patent Application Kokai (Laid-Open) No. 27,808/77, states that "this function is to enable fibers of a pulp and the like to be dispersed in water, and the most important point is whether or not the mucilaginous material added only in a small amount to a fiber slurry in sheet formation can sufficiently control the rate of drainage; the practical usefulness of a mucilaginous material in controlling the rate of drainage is evaluated in terms of anti-freeness which corresponds to the spinnability of an aqueous solution of said mucilaginous material, said spinnability corresponding also to the amount of mucilaginous material used per unit weight of pulp in practical papermaking". The values of both spinnability and anti-freeness, which are defined hereinafter, increase with an increase in purity of AMPS used as a starting material. The larger the said values, the higher the molecular weight of the copolymer and the more favorable the effect on papermaking.
In purifying AMPS by recrystallization from a solvent, the type of solvent-purification methods is limited because AMPS is soluble in only water, lower alcohols and dimethylformamide. Moreover, water tends to cause polymerization and decomposition of AMPS at temperatures 50.degree. C. or more, while dimethylformamide is disadvantageous in that precipitation of crystals of AMPS is not easy and owing to its high boiling point the drying of precipitated crystals requires a long period of time. For these reasons, methanol seems to have been chiefly used as the solvent in conventional purification of AMPS by recrystallization, as described, for example, in British Pat. No. 1,090,779 and Japanese Patent Publication 30,059/75 in connection with compounds analogous to AMPS.
However, when using purified AMPS, which is obtained by a recrystallization method in which methanol is used as the solvent (hereinafter referred to simply as methanol method), the spinnability and anti-freeness of a copolymer thereof with acrylamide do not exceed certain values, even though the AMPS is repeatedly recrystallized. Moreover, in order to enhance the purity, it is important to wash the recrystallized product, and when it is intented to dissolve, for example, AMPS in methanol at the boiling point of methanol, recrystallize the solution at 20.degree. C. and then wash the resulting crystals with the methanol to remove the mother liquor sufficiently, the final yield of crystals becomes as low as about 50% because the solubility of AMPS in methanol at room temperature is great. Further, methanol readily absorbs water because of its high hygroscopicity during the separation of crystals and other treatments. Since methanol of a high water content causes polymerization and decomposition of AMPS during purification, the methanol recovered for reuse must be sufficiently purified.